1748. Genomic Epidemiology of USA300 MRSA in Intensive Care Units (ICUs) using Whole Genome Sequencing (WGS)
Session: Oral Abstract Session: MRSA Prevention and Epidemiology
Saturday, October 29, 2016: 9:15 AM
Room: 288-290

Background:  MRSA is a significant pathogen in ICUs; healthcare worker (HCW) contamination may lead to cross-transmission.  USA300 has emerged as an important community MRSA strain; we used WGS to characterize spread in ICUs.

Methods:  From 9/2015-2/2016, unique HCWs caring for patients on MRSA contact precautions were enrolled in 4 ICUs.  Patients could be included twice.  Samples were collected from patient body sites and high-touch surfaces in patient rooms. HCW hands, gloves, and personal protective equipment were sampled pre- and post-patient encounter.  Isolates underwent WGS, with sequences mapped to reference genomes; we report only those inferred to be USA100 or USA300. 

Results:  There were 308 MRSA isolates sequenced (37% USA300, 55% putative USA100) from 62 patient encounters.  There was substantial between-encounter genetic diversity (>50 SNVs) for both USA100 and USA300 strains.  In most instances, isolates from a single patient encounter were either USA100 or USA300 (Figure 1,2) and within an encounter, isolates from the patient, HCW, and environment were genetically very similar.  In 6 (10%) encounters both USA300 and USA100 were isolated.  There were 3 USA100 and 1 USA300 cluster that contained genetically similar strains (<10 SNVs) within that cluster, yet isolates were from different patients and HCWs.  One of these clusters (Figure 2, arrow) included an isolate from a blood pressure cuff, computer keyboard, and HCW stethoscope; these isolates were collected over 6 weeks from different patient rooms but in the same ICU.  77% and 55% of all encounters with USA300 and USA100, respectively, occurred within 4 days of hospitalization (p=0.2).  A trend also suggested that HCWs were more likely to be contaminated with USA300 (13/25, 52%) than USA100 strains (13/37, 35%), (p=0.2).

Conclusion:  Patient, environmental, and HCW isolates were genetically very similar within patient encounters for both USA300 and USA100 strains, but differed between encounters, suggesting that in these ICUs, most MRSA occurrences were endogenous; in contrast, 4 WGS-defined clusters of isolates suggested spread of strains within the ICU.  Defining the relative roles of endogenous and exogenous occurrence of MRSA can help to prioritize and direct infection control efforts.



Kyle J Popovich, MD MS1,2, Stefan Green, PhD3, Koh Okamoto, MD1, Yoona Rhee, MD, ScM1, Mary K Hayden, MD1, Robert A. Weinstein, MD1,2 and CDC Prevention Epicenters Program, (1)Rush University Medical Center, Chicago, IL, (2)Stroger Hospital of Cook County, Chicago, IL, (3)Univ. of Illinois at Chicago, Chicago, IL


K. J. Popovich, None

S. Green, None

K. Okamoto, None

Y. Rhee, None

M. K. Hayden, None

R. A. Weinstein, None

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